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S. Vyshnavi , R. Islam , P. Srivastava , D. K. Misra

Abstract

Soil profiles are generally developed on stable landform in an erosion limited area with the interaction of water (rainfall). The development of the soil profile on granodioritic lithology at Anini village of Arunachal Pradesh, at an altitude of 1644 m in the NE Himalaya provides us some opportunity to understand the elemental behavior in the highly eroded and humid region, where rainfall is as high as ~2600 mm/a. The study area is geodynamically active resulting in many landslides and other mass movements. Therefore, physical and chemical weathering processes, have played a major role in its geomorphic development. The exposed least altered rock is noted with high features indicating the play of tectonics (physical weathering), which further promotes chemical weathering due to water-rock interaction substantiated by the increase of loss of ignition (Lo1) percentage from least altered rock to regolith. Due to rain water interaction, the profile shows the change of pH from alkaline to acidic, i.e., from least altered rock (pl-l-9) to regolith (pH-5). Depletion of major oxides such as SiO,, MgO, CaO, Na,O, P₂0, and enhancement of TiO,, Al,O_ Fe₂O, l(,O are noticed in the profile. Continuous enhancement of chemical index of alteration (CIA) is observed from least altered rock to regolith (56-85) with the progression of weathering. Chemical index of alteration (CIA), chemical index of weathering (CIW) and plagioclase index of alteration (PIA) of soil profile show identical behaviour in sap rock and saprolith, however during the advancement of weathering, the leaching of Ko resulted in deviation of CIW and PIA value from CIA parameter. Trace elements show enrichment from least altered rock to regolith (except Sr). The study infers that the variable geochemical proxies in soil profile are resultant of high precipitation in the tectonically active northeastern Himalaya.

Keywords

Soil Profile, Northeastern Himalaya, Geochemical Proxies, Climate-tectonic Interaction

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S. Vyshnavi ¹, R. Islam ¹, Y. P. Sundriyal ²

Affiliations
1 Wadia Institute of Himalayan Geology, Dehradun 248 001, IN
2 H.N.B. Garhwal University (Srinagar), Srinagar 246 174, IN

Abstract

Soil profiles are rarely preserved in the Himalaya due to active tectonics and erosion. We have studied two rarely well-preserved soil profiles developed on metavolcanic rocks namely Alaknanda soil profile (ASP) and Bhilangna soil profile (BSP) in Alaknanda and Bhilangna watersheds of the Garhwal Himalaya. Geochemical studies were carried out to understand the elemental mobility with reference to the least altered rock (LAR) in both the profiles and are compared. Differences in major element behaviour noticed are depletion of Ca and K in ASP, and depletion of Ca and Na in BSP. Trace elements also show variable mobility such as leaching of Rb, U and enrichment of Sr, Ni in ASP. In BSP, behaviour of these elements is just the opposite. Accumulation of ΣREEs in saprolitic layer and depletion in regolith of ASP suggest that rare earth element (REE) mobility took place during advanced stages of weathering. In BSP, increase in REE content from LAR to regolith suggests dominance of chemical weathering over physical weathering. This is also reflected in chemical index of alteration values which suggest variation of climatic parameters such as rainfall in the region.

Keywords

Climate–Tectonic Interaction, Geochemical Behaviour, Lesser Himalaya, Physical and Chemical Alterations, Weathering.

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